We are now—like it or not—immersed in a real debate about the nature of Islam. The background of deceased Boston bomber Tamerlan Tsarnaev is forcing us into it. There is no doubt Tamerlan, the elder brother of the two perpetrators, was transformed by his relatively recent embrace of radical Islam.

And so, we have the very difficult question facing us in regard to Islam: Is the propensity to terror and jihad radical in the deepest sense of word’s origin in Latin, radix, “root”? Is there something at the root of the Quran itself and the essential history of Islam that all too frequently creates the Tsarnaev brothers, Al-Qaeda, Osama bin Laden, Mohamed Atta, the Muslim Brotherhood, and Hamas, or is there some other source, quiet accidental to Islam?

That question must be taken seriously, very seriously.

I am not going to answer that question, but rather pose another: Why do liberals have so much difficulty even allowing that very serious question to be raised?

The answer to this second question is important for the obvious reason that, if liberals won’t allow the first question to be asked, then it surely can’t be answered.

A lot hangs on not answering it, in pretending it is not a legitimate question to raise. If Islam has a significant tendency to breed domestic Islamism—not everywhere, not in every case, but in a significant number of cases—then the current administration’s obsession with, say, Tea Party terror cells is woefully misplaced.

So what is it about liberalism that makes it so difficult for it to take a clear, critical look at Islam, even while liberals have no problem excoriating Christians for every imaginable historical evil?

I believe I can give at least a partial answer, if we take a big step back from the present scene and view the history of Western liberalism on a larger scale.

Liberalism is an essentially secular movement that began within Christian culture. (InWorshipping the State, I trace it all the way back to Machiavelli in the early 1500s.) Note the two italicized aspects: secular and within.

As secular, liberalism understood itself as embracing this world as the highest good, advocating a self-conscious return to ancient pagan this-worldliness. But this embrace took place within a Christianized culture. Consequently liberalism tended to define itself directly against that which it was (in its own particular historical context) rejecting.

Modern liberalism thereby developed with a deep antagonism toward Christianity, rather than religion in general. It was culturally powerful Christianity that stood in the way of liberal secular progress in the West—not Islam, Buddhism, Hinduism, Shintoism, Druidism, etc.

And so, radical Enlightenment thinkers like Voltaire rallied his fellow secular soldiers with what would become the battle cry of the eighteenth-century Enlightenment: écrasez l’infâme, “destroy the infamous thing.” It was a cry directed, not against religion in general, but (as historian Peter Gay rightly notes) “against Christianity itself, against Christian dogma in all its forms, Christian institutions, Christian ethics, and the Christian view of man.”

Liberals therefore tended to approve of anything but Christianity. Deism was fine, or even pantheism. The eminent liberal Rousseau praised Islam and declared Christianity incompatible with good government. Hinduism and Buddhism were exotic and tantalizing among the edge-cutting intelligentsia of the 19th century. Christianity, by contrast, was the religion against which actual liberal progress had to be made.

So, other religions were whitewashed even while Christianity was continually tarred. The tarring was part of the liberal strategy aimed at unseating Christianity from its privileged cultural-legal-moral position in the West. The whitewashing of other religions was part of the strategy too, since elevating them helped deflate the privileged status of Christianity.

And so, for liberalism, nothing could be as bad as Christianity. If something goes wrong, blame Christianity first and all of Western culture that is based upon it.

This view remains integral to liberalism today, and it affects how liberals treat Islam.

That’s why liberals are disposed to interpret the Crusades as the result of Christian aggression, rather than, as it actually was, a response to Islamic aggression. That’s why Christian organizations are regularly maltreated on our liberal college campuses while Islamic student organizations and needs are graciously met. And the liberal media—ever wonder why you didn’t hear last February of the imam of the Arlington, VA mosque calling for Muslims to wage war against the enemies of Allah? Nor should we wonder why, for liberals, contemporary jihadist movements in Islam must be seen as justified reactions to Western policies—chickens coming home to roost. Or when a bomb goes off, that’s why a liberal must hope that it was perpetrated by some fundamentalist patriotic Christian group.

What liberals do not want to do is take a deep, critical look at Islam. To do so just might question some of their most basic assumptions.